The present invention relates in general to a termination circuit for use in telephone voice frequency transmission systems. More specifically, the present invention relates to an active DC and AC termination circuit for a subscriber end of a voice frequency telephone transmission line. The circuit of the present invention provides all necessary functions required for this type of termination, such as proper DC loop closure impedance, sufficient longitudinal balance, proper AC termination impedance to meet echo return loss specifications, etc.
Prior art subscriber line interface circuits (SLIC) are well known. The subscriber line interface circuit is a central office interface to a two wire line. The most important function of a SLIC is to feed the current to a subscriber in order to provide power for a subscriber's equipment operation. Other important functions of the SLIC include alerting subscribers equipment by ringing, detecting subscribers equipment going off-hook as a request for service, etc. The circuit of the present invention relates to a SLIC only in that it applies to the subscriber end of the telephone line in general, and to the subscriber being a central office terminal in particular. All functions of the central office terminal are complimentary to the SLIC, that is, while the SLIC feeds the current, the central office terminal terminates or sinks the current. The SLIC and the central office terminal compliment each other, but they are entirely different circuits, having different functions with different architecture and requirements, etc.
In general several important differences exist between subscriber line interface circuits and the semi-floating active termination circuit of the present invention. For example, the semi-floating active termination circuit is a terminating/current sinking circuit rather than a current feeding circuit; DC termination consists entirely of a passive/resistive means, which is not true for a SLIC; AC termination circuits of the present invention contain no feedback to tip and ring line terminals; AC termination is determined by passive components only with active components providing virtual ground; DC termination is truly floating in reference to ground, which is not true for a SLIC; the receive amplifier for the present invention circuit must be AC coupled, while it is DC coupled for a SLIC; longitudinal balance control in the circuit of the present invention is achieved by providing a very high impedance for longitudinal current which is opposite to a very small impedance used in the SLIC circuit; and DC line current does not flow through any active device of the circuit of the present invention, whereas it does flow through active devices in the SLIC.
A typical prior art subscriber termination circuit is shown in FIG. 1 of the drawings and will be discussed in more detail below. The prior art subscriber termination circuit has a transformer which provides an AC termination by transferring proper impedance from a secondary winding to the primary winding connected across the tip and ring terminals at the voice frequency transmission line. A DC path is provided by an electronic inductor in series with a loop closure relay. An AC and DC path is blocked by an inductor L1 and a capacitor Cl, respectively. The major drawback with this prior art circuit is the high cost of materials involved, the large PC board space required and the extensive manual labor necessary for assembly of the circuit. The present invention overcomes these drawbacks of the prior art.